Kofi Annan, the joint special envoy to Syria of the United Nations and the Arab League, speaks to
reporters Thursday in Geneva, Switzerland, about his decision to step down. MARTIAL TREZZINI • AP

BEIRUT, lebanon

Kofi Annan announced his resignation Thursday as peace envoy to Syria and issued a blistering critique of the world powers, bringing to a dramatic end a frustrating six-month effort that failed to achieve even a temporary cease-fire as the country plunged into civil war.

Annan also had harsh words for the Syrian regime, saying it was clear President Bashar Assad “must leave office.”

As the violence escalated on the ground, rebels used a captured tank to shell a military air base near Aleppo — one of the first known uses of heavy weapons by the insurgents.

Speaking to reporters in Geneva, Annan blamed the Syrian government’s intransigence, the growing militancy of Syrian rebels and a divided U.N. Security Council that failed to forcefully back his effort. Since he took on the job, Russia and China have twice used their veto power to block strong Western- and Arab-backed action against Assad’s regime.

The White House said Annan’s resignation highlighted the failure of Russia and China to support action against Assad and called the regime’s continued violence against its own people “disgusting.”

“It is impossible for me or anyone to compel the Syrian government and also the opposition to take the steps to bring about the political process,” said Annan, a Nobel Peace Prize laureate and former U.N. secretary-general.

“You have to understand: As an envoy, I can’t want peace more than the protagonists, more than the Security Council or the international community for that matter.”

Annan singled out the regime for blame for the violence. But he also said the opposition’s increasing militarization had contributed to dooming his six-point peace plan, which included a cease-fire and a Syrian-led political process to end the crisis.

“The bloodshed continues, most of all because of the Syrian government’s intransigence, and continuing refusal to implement the six-point plan, and also because of the escalating military campaign of the opposition — all of which is compounded by the disunity of the international community,” he said.

“At a time when we need — when the Syrian people desperately need action — there continues to be finger-pointing and name-calling in the Security Council.”

U.N. Secretary-General Ban Ki-moon said he accepted the resignation with deep regret, adding that the search was under way for a successor to Annan, who will stay on until Aug. 31.

During his six-month tenure, Annan managed, at least in theory, to get world powers behind his plan — including the Syrian government and its allies in Iran, Russia and China — although his appeals for peace were ignored on the ground.

The regime and the rebels blamed each other for the violence, and Russia and China said attempts to sanction the regime ignored violations by the opposition.

Secretary of State Hillary Rodham Clinton praised Annan for working “tirelessly” to try to end the bloodshed in Syria. “Unfortunately, the Security Council was blocked from giving him key tools to advance his efforts,” she said.